This invention relates to methods for addressing electro-optic materials. More specifically, this invention relates to such methods in which an electrically charged fluid is used to carry charge on to the electro-optic material.
The entire disclosures of all patents and patent applications mentioned hereinafter are incorporated herein by reference.
Electro-optic displays comprise a layer of electro-optic material, a term which is used herein in its conventional meaning in the art to refer to a material having first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material. The optical property is typically color perceptible to the human eye, but may be another optical property, such as optical transmission, reflectance, luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.
One type of electro-optic display is a rotating bichromal member type as described, for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761; 6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791 (although this type of display is often referred to as a xe2x80x9crotating bichromal ballxe2x80x9d display, the term xe2x80x9crotating bichromal memberxe2x80x9d is preferred as more accurate since in some of the patents mentioned above the rotating members are not spherical). Such a display uses a large number of small bodies (typically spherical or cylindrical) which have two or more sections with differing optical characteristics, and an internal dipole. These bodies are suspended within liquid-filled vacuoles within a matrix, the vacuoles being filled with liquid so that the bodies are free to rotate. The appearance of the display is changed to applying an electric field thereto, thus rotating the bodies to various positions and varying which of the sections of the bodies is seen through a viewing surface.
Another type of electro-optic medium is an electrochromic medium, for example an electrochromic medium in the form of a nanochromic film comprising an electrode formed at least in part from a semi-conducting metal oxide and a plurality of dye molecules capable of reversible color change attached to the electrode; see, for example O""Regan, B., et al., Nature 1991, 353, 737; and Wood, D., information Display, 18(3), 24 (March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845. Nanochromic films of this type are also described, for example, in International Applications Publication Nos. WO 98/35267 and WO 01/27690, and in copending Applications Serial Nos. 60/365,368; 60/365,369; 60/365,385 and 60/365,365, all filed Mar. 18, 2002, Applications Serial Nos. 60/319,279; 60/319,280; and 60/319,281, all filed May 31, 2002; and Application Serial No. 60/319,438.
Another type of electro-optic display, which has been the subject of intense research and development for a number of years, is the particle-based electrophoretic display, in which a plurality of charged particles move through a suspending fluid under the influence of an electric field. Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. Nevertheless, problems with the long-term image quality of these displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation have recently been published describing encapsulated electrophoretic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a liquid suspension medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. Encapsulated media of this type are described, for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773; 6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,241,921; 6,249,271; 6,252,564; 6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989; 6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790; 6,422,687 and 6,445,374; U.S. Patent Applications Publication Nos. 2001-0045934; 2002-0018042; 2002-0019081; 2002-0021270; 2002-0053900; 2002-0060321; 2002-0063661; 2002-0063677; 2002-0090980; and 2002-106847; and International Applications Publication Nos. WO 97/04398; WO 98/03896; WO 98/19208; WO 98/41898; WO 98/41899; WO 99/10767; WO 99/10768; WO 99/10769; WO 99/47970; WO 99/53371; WO 99/53373; WO 99/56171; WO 99/59101; WO 99/67678; WO 00/03349; WO 00/03291; WO 00/05704; WO 00/20921; WO 00/20922; WO 00/20923; WO 00/26761; WO 00/36465; WO 00/36560; WO 00/36666; WO 00/38000; WO 00/38001; WO 00/59625; WO 00/60410; WO 00/67110; WO 00/67327 WO 01/02899; WO 01/07691; WO 01/08241; WO 01/08242; WO 01/17029; WO 01/17040; WO 01/17041; WO 01/80287; WO 02/07216; WO 02/45061; WO 02/47363; and WO 02/057,843.
Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called polymer-dispersed electrophoretic display in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, WO 01/02899, at page 10, lines 6-19. See also copending application Ser. No. 09/683,903, filed Feb. 28, 2002, and the corresponding International Application PCT/US02/06393. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.
An encapsulated electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word xe2x80x9cprintingxe2x80x9d is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.
A related type of electrophoretic display is a so-called xe2x80x9cmicrocell electrophoretic displayxe2x80x9d. In a microcell electrophoretic display, the charged particles and the suspending fluid are not encapsulated within microcapsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, International Applications Publication No. WO 02/01281, and published U.S. Application No. 2002-0075556, both assigned to Sipix Imaging, Inc.
Other types of electro-optic materials, for example, liquid crystals, especially polymer-dispersed liquid crystals, may also be used in the present invention.
In addition to the layer of electro-optic material, an electro-optic display obviously requires some means for producing the electric field needed to write on (i.e., change the optic state of) the electro-optic material. Hitherto, there have been two main groups of methods for writing on electro-optic materials. In the first group, the electro-optic material is permanently sandwiched between two electrode layers, as, for example, in the liquid crystal displays (LCD""s) used in portable computers. In most such displays, one or both of the electrode layers are patterned to define the pixels of the display. For example, one electrode layer may be patterned into elongate row electrodes and the other into elongate column electrodes running at right angles to the row electrodes, the pixels being defined by the intersections of the row and column electrodes. Alternatively, and more commonly, one electrode layer has the form of a single continuous electrode and the other electrode layer is patterned into a matrix of pixel electrodes, each of which defines one pixel of the display.
In the second group of writing methods, the electro-optic material is not permanently attached to the electrodes but is passed through a printer comprising a pair of electrodes between which the electro-optic material is passed to write an image thereon. See, for example, U.S. Pat. Nos. 6,081,285; 6,064,410; 6,045,955; and 5,866,284. Not surprisingly, many methods of this type are variations of conventional electrophotographic (xerographic) methods for printing on paper; instead of forming an imagewise distribution of charge on a photoconductor, using this imagewise charge distribution to form a corresponding imagewise distribution of toner on the photoconductor and then transferring this imagewise distribution of toner to form an image on paper, the imagewise distribution of charge can be used directly to form an image on electro-optic material.
There is a small third group of prior art methods, which may be regarded as hybrids of the two main groups discussed above. In this third group, one electrode is permanently attached to the electro-optic material, while a second electrode has the form of a stylus or similar movable member which is passed over the surface of the electro-optic material to form the desired image.
Each of the aforementioned groups of writing methods has certain problems. The first group, in which both electrodes are permanently attached to the electro-optic material, requires electrode arrays as large as the display itself and, at least in the case of high resolution displays, complicated electrical circuitry to apply the necessary charges to the arrays of electrodes needed. In the case of electro-optic materials, such as electrophoretic media, which have no threshold for switching between display states, it is necessary to use an active matrix driving technique in which each pixel of the display is provided with a separate transistor or similar non-linear device, and conventional methods for forming such arrays of non-linear devices are costly. Furthermore, although many electro-optic materials can be made in the form of paper-like sheets, conventional arrays of non-linear devices are much less flexible. Thus, although this group of methods is useful for forming small rigid displays, it is less useful for large area or flexible displays.
The second group of writing methods, in which the electro-optic material is moved through a printer, may experience problems with mechanical wear and tear of the electro-optic material caused by contact with the electrodes; while such wear and tear is negligible during the single pass of paper through a normal printer, it becomes a substantial problem when an electro-optic material is subjected to thousands of writings during its operating lifetime. Also, in this group of methods, problems may be experienced in maintaining satisfactory electrical contact between the electro-optic material and the electrodes, and variations in contact resistance may manifest themselves as artifacts in the final image. Finally, this group of methods are susceptible to edge effects caused by variation in electric field strength as the electro-optic material moves into and out of the field between the electrodes. Consider, for example, a bichromal ball electro-optic material comprising a large number of half-black, half-white spheres each having an internal dipole and rotatable within cavities in a matrix material. In theory, if one pixel of such a material is to be written black by passing the material through a print head, the head should apply an electric field perpendicular to the plane of the electro-optic material with a polarity such that the black hemispheres face a viewing surface of the material and no portion of the white hemispheres is visible. Such a perpendicular field may be applied at a central portion of the print head, but as the pixel leaves the print head it may experience an area in which the electric field is not precisely perpendicular, especially if the print head has to write the next pixel white. In such an area, the balls will tend to rotate so as to align their dipoles with the non-perpendicular field, with the result that after the electro-optic material has left the print head a small proportion of the white hemispheres will be visible in the supposedly black pixel. Similarly, a small proportion of black hemispheres may become visible in the supposedly white pixel. The net effect of these edge effects is an undesirable loss in contrast in the printed image.
The present invention provides a first process for addressing an electro-optic material which does not require the provision of electrodes permanently secured to the material, but which avoids the problems of establishing good but temporary electrical contact between the electro-optic material and an electrode in a printer. Essentially, the present method makes use of an electrically charged fluid to charge a surface of the electro-optic material and thus write thereon.
The present invention also provides a second process for addressing an electro-optic material which permits a non-conductive stylus to be used in the third group of writing methods discussed above.
In one aspect this invention provides a process for addressing an electro-optic material having first and second display states differing in at least one optical characteristic and being capable of being changed from its first to its second display state by application of an electric field to the material, the process comprising applying an electrically charged fluid to a portion of at least one surface of the material, thereby changing the display state of a portion of the material.
In a second aspect this invention provides a process for addressing an electro-optic material having first and second display states differing in at least one optical characteristic and being capable of being changed from its first to its second display state by application of an electric field to the material, the process comprising contacting the electro-optic material with a non-conductive brush means wet with a conductive liquid while applying a potential difference between the brush means and the electro-optic material.